Fluoro-refractory coatings have been widely used in metal surface treatment of improved corrosion inhibition and improved adhesion of a subsequently applied paint layer. Fluoro-refractory coatings are applied through chemical reactions between the metal and the bath solution which converts or modifies the metal surface into a thin film with required functional properties. Fluoro-refractory coatings are particularly useful in surface treatment of metals such as steel, zinc, aluminum and magnesium.
A common example of fluoro-refractory coatings is a chromium conversion coating. Chromium conversion coatings are typically applied in any one of a number of processes using a dilute aqueous acid solution of a chromate complex followed by a water rinse. Most often, the dilute aqueous acid solution contained hydrofluoric acid (HFl). Generally, fluoro-refractory coatings containing hafnium, zirconium or titanium are applied in the same manner.
It has been discovered, as fluoro-refractory coatings fail, that the corrosion products associated with the failure contain a higher concentration of fluorine than the coating as initially applied. To the best of the knowledge of the inventors of the present application, the following observation is best explained as follows. Fluoro-refractory coatings formed on substrates, such as aluminum, etc., absorb a quantity of fluorine when the fluoro-refractory coating is applied as a result of incomplete hydrolysis of precursors, such as potassium or sodium fluorozirconate or fluorotitanate, fluorozirconic or fluorotitanic acid, or from substrate reaction products such as aluminum fluoride. During operation of the coated article, fluorine slowly leaches out of the coating and reacts with ambient water to form hydrofluoric acid, HF. Being corrosive, HF begins eroding the fluoro-refractory coating thus causing pitting and general corrosion, etc. and exposing the substrate to the atmosphere and elements. Such exposure in turn significantly shortens the useful lifecycle of the article, that is, a part, e.g., a gas turbine engine.
Therefore, there exists a need for a process for extracting fluorine from a fluoro-refractory coating.